Dynamic loads on prefabricated modular building unit during road transportation

Abstract

Prefabricated modular construction is set to revolutionize the modern construction industry once again. Over the past couple of centuries, prefabrication has proven to be a more cost-efficient and time-saving alternative to conventional design practices. Computer-aided design has facilitated more complex projects in terms of architecture, planning and cost. Prefabricated construction has benefitted from the advent of building information modelling (BIM), 3-D printing, robotics, and assembly line automation. What is poised to become a multi-trillion-dollar industry in the coming decade, the modular construction is still navigating through its bottlenecks around different parts of the world.

Benefits of doing lesser at the construction site and more at the offsite are pushing the frontiers of prefabricated modular construction every day. The demand for pre-furnished, all equipped plug-in ready modules has increased. With many issues pertaining to skilled labour availability, shipping, transportation, and erection issues, structural robustness, and project economics, yet to be resolved, the industry demand for prefabrication is ever increasing.

The research undertaken in this project focused on quantifying the hazards due to road induced vibrations to the modular building unit and its internal components. A stepwise approach of the project quantified the vertical acceleration spectra for non-structural components inside the modules. The first part addressed the continuous vibrations sustained by the modular unit and the internal components due to road roughness. The second part calculated the impact-induced hazard by predicting the risk of the vertical separation between the module and the trailer-bed. Stochastic modelling was employed using stationary processes to create a large sample of road profiles consistent with the required roughness power spectral density functions. Multi-degree of freedom lumped mass model was developed to predict the truck trailer's response to the road profile roughness in the vertical plane. The truck-road numerical modelling was validated with field experiments on known road surfaces. Response acceleration spectra for the components attached to the modules carried by trailer were reported as the sustained dynamic actions on the cargo. A probabilistic estimate for vertical gap formation (and vertical impact velocity) between the strapped cargo (module) and the trailer-bed was made by extending the truck-road interaction model. The impact between the air-borne edge of the box module structure with that of the trailer-bed was studied analytically, numerically and experimentally. Component response acceleration spectra were derived for such a pounding scenario considering different masses of the modules impacting with different velocities. Design acceleration spectra for components were broadly classified into three frequency regimes (< 20 Hz, < 50 Hz, > 50 Hz) stipulating vertical acceleration demand of the non-structural component fixtures inside modules during transportation. Structural guidelines available for the component fixture design could only be found in seismic standards. What is typically sufficient for component design in conventional buildings may not be so for furnished modular structures as the transportation induced dynamic loads could be much higher than the seismic provisions for the non-structural components.

This study offers guidelines for the capacity design of the internal fixtures of a fully furnished modular structural unit through numerical analysis backed by experimental tests. The findings of this study are expected to assist the engineers in designing the building utilities, suspended ceilings, non-structural walls, wall-mounted equipment of a modular building unit for transportation induced loads.

Recommendations: 1. Incorporate higher design forces for internal component fixtures governed by the acceleration response spectra derived in the study. 2. Provide rigid latching between the MBU and the trailer-bed similar to that of the container shipping module. 3. In the case of larger MBUs that cannot be rigidly fastened to the trailer-bed, increase vertical tie strength to 40% of the total weight of the cargo to avoid vertical separation. 4. Incorporate softer palleting materials: a. To filter out high-frequency vibrations b. To alleviate the damage hazard during pounding should it occur. 5. Increase the support locations for the MBU by increasing the number of mounts/pallets used during transportation. 6. Orient the MBU on the trailer-bed such that vibration-sensitive components would lie in the front portion of the trailer.